This invention relates to the preparation of barrier coatings on films and particularly to oxygen, carbon dioxide, flavor and aroma barrier coatings on films for use in packaging.
The pharmaceutical and food industries have, over recent years, increasingly provided products in a prepackaged form. Fruit and vegetables for example, apples and tomatoes, meat and cheese are often prepackaged in a tray and the tray and the fruit are covered with a transparent film.
One of the most important requirements for films used for packaging applications is that they should protect products from aromas or odors in the vicinity in which the products are stored, i.e. they should act as barriers to such aromas or odors. Similarly the films are utilized as barriers to prevent strong smelling products contained in packages from tainting the surrounding area with their aroma during storage.
Oxygen barrier coatings are utilized to prevent the ingress of oxygen into products with a view to extending the shelf life of products and carbon dioxide barrier coatings are typically utilized to prevent the release of carbon dioxide from rigid plastic bottles holding carbonated drinks.
U.S. Pat. No. 5,215,822, describes a process of controlling the impermeability of a film to gases by mixing a vinyl benzylamine silane with an ethylenically unsaturated carboxylic acid e.g. itaconic acid, in a solvent, solubilising, hydrolyzing and equilibrating the resultant solution and coating this solution on a corona treated low density polyethylene film and drying the resulting film. The coated film is then subjected to an electron beam radiation to graft the coating to the film surface and further improve the barrier properties of the silane coating. The vinyl benzyl amine silane was also co-polymerised with 3-(2-aminoethyl)-aminopropyl trimethoxy silane or gamma aminopropyltriethoxysilane prior to mixing with the acid. The resultant mixture was then used to coat the relevant substrate. While these coatings gave excellent gas barrier properties at low to moderate relative humidity values, the gas permeability was less satisfactory at very high relative humidity values. In addition, the use of electron beam radiation may lead to cross-linking or chain scission in underlying plastics substrates, with concomitant loss of tensile properties. The use of electron beam radiation and several other complicated and/or expensive procedures with respect to coatings of this type containing mono-silyl constituents can make the use of such compounds for the manufacture of coated films for use in the packaging industry unattractive.
U.S. Pat. No. 4,689,085/U.S. Pat. No. Re. 34675 describes the preparation of disilylated hydrocarbons of the general structure (RO)3SiRxe2x80x2Si(OR)3 where OR is a hydrolysable group and Rxe2x80x2 is a divalent organic radical. They further teach the combination of a silane coupling agent for example, 3-methacryloxypropyltrimethoxysilane with a disilyl cross-linking compound of the general formula structure (RO)3SiRxe2x80x2Si(OR)3 where OR is a hydrolysable group and Rxe2x80x2 is a divalent organic radical for example, an alkylene group or a selection of branched, unsaturated or aryl substituted groups as an improved coupling agent and primer mixture demonstrating that the combination of the components provided improved results compared with either component when used alone.
These compositions are said to be useful as primer coatings between non-particulate surfaces and polymer coatings as pretreatments for particulate fillers before compounding, and as additives to filled polymer systems during compounding as well as for treating glass cloth.
Various proposals to employ organosilicon compounds in the treatment of plastics films to achieve gas barrier properties have been made. However, bis-silanes have not previously been utilized to control the gas barrier properties of a substrate. Furthermore, it is to be noted that none of the prior art discussed above suggests the use of such compounds for coating materials to be cured on plastic, cellulosic, glass or metal substrates to achieve gas barrier properties.
It is one of the various objects of the present invention to provide a process for treating a surface of a substrate to provide improved barrier properties.
The present inventors have now surprisingly found that substrates having coatings of selected moisture cured disilylated secondary amines demonstrate excellent gas barrier properties at low to high relative humidity values. Furthermore, these properties may be achieved without the need for exposure of the coating to electron beam or other forms of ionizing radiation.
The present invention provides, in accordance with one of its aspects, a process for treating a surface of a substrate with a compound of the general formula
RaX3-aSixe2x80x94Zxe2x80x94SiX3-aRa
wherein Z is Rxe2x80x2NH(Rxe2x80x2NH)pRxe2x80x2, each R is selected from the group consisting of a hydrogen atom and a hydrocarbon group, each X is selected from the group consisting of an alkoxy group with 1 to 4 carbon atoms, a halogen atom, an oxime group or an acyloxy group, each Rxe2x80x2 is a divalent hydrocarbon group having 1 to 12 carbon atoms; a is from 0 to 3 and p is 0 or 1; which process comprises applying the compound on to the substrate to form a layer and exposing the layer to moisture.
In a process according to the present invention there is used a compound of the general formula
RaX3-aSixe2x80x94Zxe2x80x94SiX3-aRa
wherein Z is Rxe2x80x2NH(Rxe2x80x2NH)pRxe2x80x2.
In this formula each R is preferably a hydrogen atom or a hydrocarbon group having 1 to 10 carbon atoms, for example a saturated or unsaturated aliphatic or aromatic group, for example alkyl alkenyl or phenyl groups; preferred groups are methyl and ethyl, the most preferred of which are methyl groups. Each X is an alkoxy group with 1 to 4 carbon atoms, a halogen atom, an oxime group or an acyloxy group, of these methoxy and ethoxy groups are preferred, the most preferred being methoxy groups. Rxe2x80x2 may be a divalent hydrocarbon group having 1 to 12 carbon atoms, preferably each Rxe2x80x2 has from 2 to 3 carbon atoms. Each a is from 0 to 3 but is most preferably 0, and p is 0 or 1. The best results are obtained by use of compounds in which each X is a methoxy group, each Rxe2x80x2 is a propylene group, a is 0, and p is 0, i.e. when the compound is bis-(gamma-trimethoxysilylpropyl)amine.
These materials, referred to as disilylated secondary amines may be prepared by processes known in the art for example, as disclosed in U.S. Pat. Nos. 2,832,754, 2,920,095, and 5,101,055.
In a process according to the invention the surface of a substrate is treated with a compound as aforesaid with or without the addition of:
i) a solvent selected from the group consisting of an alcohol, an ether, an ester, a hydrocarbon, and water in the presence of a polybasic acid;
ii) an organic acid having two or more acid substituents wherein the organic acid is a polybasic carboxylic acid selected from the group consisting of itaconic acid, citric acid, succinic acid, butane tetracarboxylic acid, ethylene diamine tetracetic acid, ascorbic acid, tetrahydrofuran tetracarboxylic acid, cyclopentane tetracarboxylic acid, and benzene tetracarboxylic acid;
iii) a polymer or co-polymer of an unsaturated carboxylic acid selected from the group consisting of itaconic, citraconic, mesaconic, maleic, fumaric, acrylic, methacrylic, sorbic, cinnamic acid, wherein the co-polymer is a co-polymer with any appropriate unsaturated monomer selected from the group consisting of one or more other unsaturated carboxylic acids, ethylene, propylene, styrene, butadiene, acrylamide and acrylonitrile;
iv) a condensation catalyst;
v) a filler selected from the group consisting of silicone resin, silica, magnesium oxide, clay, diatomaceous earth, calcium carbonate, finely ground quartz and nanoparticles.
While the process of the present application may proceed using a solventless system, the compound may be dissolved in a solvent (i) and subsequently applied in solution. This is usually carried out with a view to reduce the total solids applied and so control coat weight during application, particularly in relation to cases where a catalyst is being used. In general, alcohols and blends thereof are suitable solvents because compounds of the present invention are soluble therein. The selected solvent must wet the substrate. Preferably, the solvent is non-toxic, and does not extend the drying time of the layer beyond a commercially acceptable period. The amount of solvent may range from about 1 to about 99 parts by weight and is preferably from about 50 to about 95 parts by weight of the total composition.
Preferred solvents (i) are alcohols for example, methanol, ethanol, n-propanol, isopropanol, butanol, and 1-methoxy-2-propanol, the most preferred solvent being methanol. Alternative solvents which may be utilized include an ether, for example ethyl ether, an ester for example ethyl acetate, a hydrocarbon for example cyclohexane, and water in the presence of a polybasic acid. It was found that a solution of the compound in water alone almost immediately formed a gel whereas an aqueous solution was stable in the presence of a polybasic acid.
The organic acid (ii) may be added to the compound whether or not water is the solvent being used and may be selected from the group consisting of itaconic acid, citric acid, succinic acid, butane tetracarboxylic acid, ethylene diamine tetracetic acid, ascorbic acid, tetrahydrofuran tetracarboxylic acid, cyclopentane tetracarboxylic acid, and benzene tetracarboxylic acid.
The polymer or co-polymer of an unsaturated carboxylic acid (iii) may be prepared from one or more unsaturated carboxylic acids selected from the group consisting of itaconic, citraconic, mesaconic, maleic, fumaric, acrylic, methacrylic, sorbic, and cinnamic acids. The co-polymers may be co-polymers with other unsaturated carboxylic acids or any other unsaturated monomer, for example, monomers selected from the group consisting of ethylene, propylene, styrene, butadiene, acrylamide and acrylonitrile.
The condensation catalyst (iv) may be used with the compound to catalyze the cure process. The compound, when used in the process of the present invention always contains at least one secondary amine group, and therefore will always self catalyze the cure reaction to some extent. However, the cure may be accelerated by use of a catalyst. Furthermore, compounds of a similar structure not having an amine group present, as used in the examples for comparative purposes, will require a catalyst to at least initiate cure. Any suitable condensation catalyst may be added, for example, tin and titanium compounds or amines may be utilized.
Any appropriate filler (v) may be added to the compound. The filler may be selected from, for example, silica, magnesium oxide, clay, diatomaceous earth, calcium carbonate, finely ground quartz, and nanoparticles. Silicon containing nanoparticles such as silicates, for example exfoliated vermiculite, montmorillonite and apophyllite, may be added to the compound in order to reduce the thickness and/or weight of the resultant coating. This would be particularly usefull if the nanoparticles were exfoliated after having been thoroughly mixed into a compound or mixture prior to applying the layer to the substrate.
In a process according to the invention the layer may be applied on to a wide variety of substrates, including, but not limited to polyolefins, including oriented polypropylene (OPP), cast polypropylene, polyethylene, polystyrene; polyolefin copolymers, including ethylene vinyl acetate, ethylene acrylic acid, ethylene vinyl alcohol (EVOH), ionomers, polyvinyl alcohol and copolymers thereof; polyacrylonitrile; polyvinyl chloride, polyvinyl dichloride, polyvinylidene chloride and polyacrylates.
Further alternative substrates include polyesters, for example, polyethylene terephthalate (PET) or polyethylene naphthalate (PEN); polyamides for example, nylon and meta-xylene adipamide (MXD6), and polyimides.
Even further possible substrates include polysaccharides, for example, regenerated cellulose, glassine or clay coated paper, paperboard or Kraft paper or metallised polymer films and vapor deposited metal oxide coated polymer films for example, AlOX, SiOX, or TiOX.
The layer applied according to the invention may be applied to the aforesaid substrates when they are in the form of a film or sheet or molding, though this is not obligatory. The substrate may be selected from a copolymer, a laminate, a blend, a coating or co-extruded or a combination of any of the substrates listed above according to the compatibility of the materials concerned with each other. In addition, the substrate may be in the form of a rigid container made from materials such as polyethylene, polypropylene, polystyrene, polyamide, PET, polymers of EVOH, or laminates containing such materials. The layer may be applied onto a substrate in any desired amount; however, it is preferred that the layer be applied in an amount suitable to form a coating weight on the substrate of from about 0.05 to about 20 g/m2. Preferably the coating weight is from about 0.5 to about 10 g/m2, and most preferably is from 0.5 to 3g/m2. Coating weights may be determined by gravimetric comparison. The layer may be applied to the substrate by any conventional process, for example, spray coating, roll coating, slot coating, meniscus coating, immersion coating, and direct, offset, reverse gravure coating, and Myer rod.
In a process according to the invention the layer is exposed to moisture and it is believed that curing of the compound occurs thereby. The layer may be exposed to heat at the same time as it is exposed to moisture. The curing process may additionally involve the application of heat in order to optimize and accelerate the cure process. Generally, the higher the temperature, the faster the coating will solidify. Furthermore, heating in the presence of moisture will accelerate both the rate of hydrolysis of silicon-alkoxy groups and the rate of condensation of silicon bonded alkoxy groups to form silicon-oxygen-silicon linkages.
The upper temperature limit for the heating step is the temperature above which the substrate will undergo an unacceptable degree of distortion. In the present invention it has been found that the layer may be dried to form a coating at any temperature from room temperature up to about 140xc2x0 C., with temperatures of from about 60xc2x0 C. to about 120xc2x0 C. being preferred and temperatures of about 90xc2x0 C. to about 110xc2x0 C. being most preferred. The time period over which the layer may be heated is, as might be expected, temperature dependent and at the most preferred temperature range referred to above the resultant coating will become tack free in a period of from 1 to 10 seconds.
In cases where a solvent is present, the heating step in a process in accordance with the invention becomes of increased importance as it not only serves as a means of accelerating the curing process but also serves as a means of evaporating the solvent in the layer.
If desired, substrates used in a process according to the invention may be pretreated prior to application of the layer, for example, by corona treatment, plasma treatment, acid treatments and/or flame treatments, all of which are known in the art. Furthermore, any of the foregoing substrates may have a primer or primers applied thereon prior to application of the layer. The primers may be applied to the substrates by any appropriate process known in the art, for example, spray coating, roll coating, slot coating, meniscus coating, immersion coating, and indirect, offset, and reverse gravure coating and extrusion coating. Suitable primers may include, but are not limited to carbodiimide, polyethylenimine, and silanes, for example, N-(2-aminoethyl)-3-aminopropyltrimethoxy silane and aminopropyltriethoxysilane.
Substrates treated by a process according to the invention may be subsequently used without further treatment. However, it is possible to bring a second substrate as described above, into contact with a first substrate under adhesive bond forming conditions, in which case the compound alone or in combination with other additives serve as a primer or adhesive. When two substrates are present, the application of the compound and the first and second substrates may be in a continuous process where the application of the compound onto the first substrate and the second substrate onto the compound occurs substantially simultaneously. Alternatively a stepwise process may be utilized wherein the layer is initially applied onto the first substrate and subsequently the second substrate is applied onto the layer.
The additional coatings may be for example, metallic top coats for example, metallised coatings using aluminum or alternatively vapor deposited metal oxide coatings of AlOX, SiOX or TiOX. Packaging requiring metallised or vapor deposition metal oxide coatings may use coatings prepared in accordance with the process of the present invention as primers. There has, for a long time, been a problem with metallised films of this type in that whilst such films provide high barrier levels with respect to gases, aroma and moisture, the metal layer itself is very often a weak point due to surface defects in and/or lack of adhesion of the metallised layer to the flexible plastic substrate.
Oxygen, carbon dioxide, aroma and flavor barrier coatings as prepared by treating substrates using a process according to the invention may be used for a wide variety of packaging containers, for example, pouches, tubes, bottles, vials, bag-in-boxes, stand-up pouches, gable top cartons, thermo-formed trays, brick-packs, boxes, cigarette packs and the like. They may also be used in any application wherein gas, or aroma barrier properties are desired, for example, tyres, buoyancy aides, inflatable devices generally.
For packaging applications where a barrier coating as opposed to an adhesive layer is utilized one of the most useful applications is where oriented polypropylene film is the substrate. By use of a process according to the invention one may provide coated substrates which have a significant barrier effect at high relative humidities e.g. 80% to meet the goals of the packaging industry i.e. coatings with oxygen transmission rates (OTR) of less than 0.00015 m3/m2/day for oriented polypropylene. For example, a 30 micron uncoated biaxially oriented, corona treated polypropylene film is generally found to have a permeability to oxygen of 0.0015 m3/m2/day as measured at ASTM D3985-81 measured at 80% relative humidity. Such films when treated in accordance with a process of the present invention can, in some instances, have an OTR of less than 0.000005 m3/m2/day.
The invention provides in another of its aspects an oxygen, odor or flavor barrier coating composition comprising a compound of the general formula:
RaX3-aSixe2x80x94Zxe2x80x94SiX3-aRa
wherein Z is Rxe2x80x2NH(Rxe2x80x2NH)pRxe2x80x2; and wherein each R is selected from the group consisting of a hydrogen atom and a hydrocarbon group, each X is selected from the group consisting of an alkoxy group with 1 to 4 carbon atoms, a halogen atom, an oxime group or an acyloxy group, each Rxe2x80x2 is a divalent hydrocarbon group having 1 to 12 carbon atoms; a is from 0 to 3 and p is 0 or 1; and one or more constituents selected from the group consisting of:
i) a solvent selected from the group consisting of an alcohol, an ether, an ester, a hydrocarbon, and water in the presence of a polybasic acid;
ii) an organic acid having two or more acid substituents wherein the organic acid is a polybasic carboxylic acid selected from the group consisting of itaconic acid, citric acid, succinic acid, butane tetracarboxylic acid, ethylene diamine tetracetic acid, ascorbic acid, tetrahydrofuran tetracarboxylic acid, cyclopentane tetracarboxylic acid, and benzene tetracarboxylic acid;
iii) a polymer or co-polymer of an unsaturated carboxylic acid selected from the group consisting of itaconic, citraconic, mesaconic, maleic, fumaric, acrylic, methacrylic, sorbic, and cinnamic acid, wherein the co-polymer is a co-polymer with any appropriate unsaturated monomer selected from the group consisting of one or more other unsaturated carboxylic acids, ethylene, propylene, styrene, butadiene, acrylamide and acrylonitrile;
iv) a condensation catalyst; and
v) a filler selected from the group consisting of silicone resin, silica, magnesium oxide, clay, diatomaceous earth, calcium carbonate, finely ground quartz and nanoparticles.
In a still further aspect of the invention there is provided the use of a compound of the general formula
RaX3-aSixe2x80x94Zxe2x80x94SiX3-a
wherein Z is Rxe2x80x2NH(Rxe2x80x2NH)pRxe2x80x2; and wherein each R is selected from the group consisting of a hydrogen atom and a hydrocarbon group, each X is selected from the group consisting of an alkoxy group with 1 to 4 carbon atoms, a halogen atom, an oxime group or an acyloxy group, each Rxe2x80x2 is a divalent hydrocarbon group having 1 to 12 carbon atoms, a is from 0 to 3 and p is 0 or 1; in or for an oxygen, odor or flavor barrier coating.
One major advantage of the present invention over recent prior art is that no ionizing radiation, such as electron beam or ultra violet radiation is required to cure the layer.
A further advantage of coatings prepared by the process of this invention is that they provide significantly improved abrasion resistance compared to uncoated substrates. In order that the invention may become more clear there now follows a detailed description of several coatings prepared in accordance with the present invention. Other coatings using a variety of aminosilanes are provided as comparative examples. The tests were carried out with respect to oxygen transmission rate (OTR) at several Relative Humidities (RH). It is to be appreciated that generally if coatings are barriers to oxygen they are also barriers to odors and flavors.
In each of the following Tables the values of OTR are given in the units of 10xe2x88x926 m3/m2/day. In all the following examples oxygen transmission rate (OTR) was determined using a MOCON(copyright) Ox-Tran 2-20 apparatus having a COULOX(copyright) coulometric sensor. Each substrate having a dried/cured layer of the compound thereon was clamped into a diffusion cell forming a divide between two chambers. Both chambers are then purged of oxygen using an oxygen free carrier gas which is usually a mixture of 3% by volume of hydrogen in nitrogen. Oxygen is introduced into the first chamber and is allowed to permeate through the sample into the second chamber wherein any oxygen molecules present are transported by the carrier gas to the sensor. The signals received at the sensor are caused by the reaction between the oxygen and hydrogen.